Microwave ovens are capable of cooking, defrosting and reheating food very rapidly but, because the heat energy is generated within the food itself by agitation of the food molecules, the exterior of the food does not reach the temperature required for browning. This disadvantage is met in known microwave ovens by providing some form of thermal heating means in addition to the microwave generator. For example, British Specifications Nos. 1172918 and 1332122 disclose ovens which have thermal heating elements in addition to microwave generators.
Known microwave ovens adopt different ways of using the two forms of energy, i.e. the microwave energy and thermal energy. One known oven produces thermal power only during a first cooking period followed by microwave power only during a second cooking period, the changeover from thermal power to microwave power occurring automatically. During the first period, the thermal power tends to crisp and brown food items without cooking them in the middle. During the second period when microwave energy only is supplied to the oven, steam is generated as a result of the middle of the food becoming heated, and this moisture must find its way out of the already crisp and brown outside of the food. The result is that the moisture tends to break down the outside of the food layer which absorbs the moisture and becomes soft and unappetizing. To disperse the moisture, the oven is vented during the second period and the temperature drops within the cavity causing deterioration in certain food products, particularly pastry items.
A second known microwave oven operates in the reverse sequence, by having a first cooking period during which microwave energy only is produced followed by a second period during which thermal energy only is produced. During the first period the oven cavity is vented by a forced flow of air to dissipate steam. On commencement of the second period, venting ceases and the thermal power is applied with the object of browning the food item.
Both of these known microwave ovens fail to deal with the problem of dissipating the moisture as it is produced during the period of production of microwave energy. Excess moisture causes condensation on cool oven surfaces as well as making the food soft and pappy. Too much venting to dissipate moisture reduces the temperature of the oven while the application of too much thermal energy tends to burn the outside of the food before the food is cooked in the middle.
The invention aims to provide a microwave oven and a method of cooking food in which the thermal heat input into the oven cavity is sufficient to cope with the moisture driven off the food as the latter is cooked by microwave energy. To achieve this, it has been found that continuous microwave power should be applied simultaneously with continuous thermal power.
Microwave ovens are known in which microwave power is applied simultaneously with thermal power. In general, however, such ovens have been large commercial or industrial ovens with high power ratings. This has meant that the power needed for the microwave generator and for the thermal heating elements for any cooking operation has hardly been limited by a power input level to the oven. By contrast, the invention is concerned with portable, plug-in appliances which are designed for domestic use and which can be plugged into a domestic socket having a specified power limit which differs from country to country. The present invention is concerned with providing an oven designed to operate with a maximum power input of 1800 watts (to suit the USA), and a oven designed to operate with a maximum power input of 3000 watts (to suit the UK).
It is known to provide microwave ovens with a fan which causes a flow of hot air (heated by thermal heating means) to pass over the food being cooked. Successful cooking of food in such microwave ovens depends on the correct choice of a number of factors, one of which is the pattern of air flow within the oven cavity. Once commercially available oven draws heated air into the cavity from the mid region of the back wall of the cavity, the air being drawn forwardly before splitting up into two streams which are turned to flow backwards along the sides of the oven cavity, before leaving the cavity at two locations in the back wall of the cavity. It has been found that this air flow pattern does not always satisfactorily dissipate moisture generated from the food as it is being cooked by the microwave power. This is a particular problem with pastry items which become soft and pappy if the moisture resulting from the cooking is not dissipated by the hot air stream.